1. Field of the Invention
The field of this invention lies within the field of bonding wires and other devices within the electronics art. More particularly, it lies within the ultrasonic bonding art for wire bond applications. It specifically relates to ultrasonic wire bonding and discrete orientation of a wire bonding tool to the work that it is to be bonded to.
2. The Prior Art
Various ultrasonic methods are utilized for welding a wire between a semiconductor device and other circuits or to the terminals of the device itself.
The foregoing welding means can be divided into two types. A first type is a wedge bonding type, while the second type is referred to as ball bonding.
Bonding in general within the framework of this art and this invention is directed to that process that uses a combination of force applied to the two surfaces that are to be welded with an attendant ultrasonic vibration. The ultrasonic vibration assists in forming a true cold weld to bond the two metal items together.
In some cases, a slightly elevated temperature is added to the process. Regardless of this, the temperature is much lower than that used in thermocompression bonding.
Ball bonding requires the user to form a ball on the end of a wire projecting through a ceramic or metal lead hole called a capillary. This is done with a fine hydrogen flame or spark discharge, so that the ball is formed at the end of the wire that is fed through the capillary.
Once the ball is formed, it is brought into contact with the surface to which it is to be welded. This is done by lowering the capillary holding the wire with the ball at the end and the transducer for bonding the device until contact is made with the surface that it is to be bonded to. After contact, a suitable pressure or force is applied to the ball at the end of the wire, holding it against that portion it is to be welded to. Ultrasonic energy is then generated to complete the bond.
Wedge bonding differs from the foregoing ball bonding to the extent that the wire is brought under a tool. No ball is used for bonding. Instead, the wire is simply pressed against the surface and ultrasonic energy is applied by the transducer to flatten the wire. The foregoing action completes the bond.
Although many mechanical differences exist in the various types of bonding tools used for wedge bonding, ball bonding, as well as other types of ultrasonic bonding for wires, there are similar problems in applying either process. One problem that is substantial is to move the interconnect wire to the surface at a high speed, yet not appreciably deform the wire, due to deceleration forces developed when the mass of the transducer is stopped by contact with the work surface. Any flattening of the wire tends to prevent a high quality bond in the flattened region.
To solve the foregoing problem, manufacturers of wire bonding machinery have tended to reduce the velocity of the transducer and bonding head just before touchdown occurs or is expected to occur. However, it is oftentimes not convenient to do this. Furthermore, many of the surfaces to which a bond is to be made may vary in height from semiconductor device to semiconductor device, or within the same device itself.
In some devices, there is a need to attach a wire to a plurality of surfaces at widely varying levels within the device itself. This particular physical problem necessitates frequent readjustment of the bonding machine and slows down production, as well as creating poor bonds.
The foregoing problem can be overcome if the point of touchdown can be sensed. In other words, if at the moment of touchdown, contact is sensed and the transducer and tool is stopped, a significantly improved bond and production can take place.
In order to do this, the transducer must have a low moment of inertia and be driven by a motor that can be stopped quickly. These are readily available in the form of a transducer and a motor that can be stopped (i.e., a DC or stepping motor). However, the remaining portion of the problem was not soluble until this invention.
The invention incorporates an ultrasonic generator and transducer with a carrier signal that is imposed thereon. When the tool and wire make contact, it creates an increase in transducer impedance and lowers the current in the transducer.
Simply stated, the transducer is driven with a sixty KHz constant voltage signal. This is the same signal used for bonding. The impedence change is monitored by measuring the value of the sixty KHz current flowing to the transducer. This current upon contact decreases significantly at the point of touchdown due to dampening.
A voltage comparator circuit monitors the DC level suitable for signalling the motor control circuit to stop the motor. Afterwards, bonding takes place and the unit is then recycled for another touchdown sensing.
As will be seen in the following description of the specification, this invention is a substantial improvement over the prior art by allowing for increased sensitivity at touchdown, preventing poor bonding, while at the same time enabling increased productivity through the elimination of certain adjustments that are required.